Seaworthy Magazine: Why Boats Sink (And How to Keep Them Afloat)

The cost of repairing a boat that has been underwater, even briefly, is usually about 40% of its value. Besides having to pay the deductible, the skipper typically loses the use of the boat for several weeks while it is being repaired. The best defense against a dockside sinking? Visit your boat. And, at least twice a season, inspect any fittings above or below the waterline that could be letting water into the boat. All too often, skippers rely on bilge pumps to bail them out when they can’t visit their boats. The pump fails and the boat sinks. If you can’t visit your boat regularly, consider using a buddy system with other boat owners to watch each other’s boats.

Modern boats sink for a variety of reasons, which is the point of this discussion. According to the BoatUS Marine Insurance claim files, for every boat that sinks underway, four boats sink in their slips. There are two reasons for this discrepancy. One reason is whenever a boat leaves the dock, someone is aboard, which leaves open the possibility that the leak will be discovered and the problem corrected before it sinks the boat. And, reason # 2, boats tend to spend a majority of their time at the dock.

Why Boats Sink at the Dock

The handsome sportfisherman had been an impressive floating beauty when the owner left the marina on Monday afternoon, barely 13 hours earlier, which is why he was having so much trouble believing that it was his boat that was on the bottom when the call came from the marina manager. There were five bilge pumps aboard, all of which were working. How could his boat have sunk so quickly?

The answer was traced to a cracked generator intake hose, which, according to the surveyor’s report, may have been leaking steadily for weeks or even months. The bilge pumps kept the water out until the batteries (and the pumps) died and the boat filled with water. BoatUS Marine Insurance claim #970083A.

When a boat sinks at the dock, the question most likely to be asked is: “What happened to the bilge pump?” That’s the wrong question, however. By dutifully emptying the bilge periodically, a bilge pump can actually hide a problem--until the pump clogs or the battery goes dead. Water, not bilge pumps, sinks boats. The correct question should be: Where did the water come from? For an answer, BoatUS Marine Insurance examined 100 claim files of boats that sank in their slips.

Where the Water Came From
Click on a link below for more information:
Reason Percentage
50%
32%
9%
8%
1%

Underwater Fittings
In 50% of the dockside sinking claims, water found its way into the bilge through leaks at underwater fittings. The majority of the leaks were at stuffing boxes (12), followed by outdrive or shift bellows (11), failed hoses or hose clamps (eight), sea strainers (four), and drain plugs (four).

There were two sinkings each from air conditioning fittings, gate valves, transducers, mounting bolts, and mufflers. One boat went to the bottom as a result of a leaking speedometer impeller. It is certainly possible that more than one fitting had been leaking.

Rain and Snow
Water falling from the sky, either rain, snow, or sleet, accounted for a whopping 32% of the sinking claims. Everybody has seen a rowboat or two awash, so this shouldn't be a surprise. What may be startling is that all of the claims involved boats with self-bailing cockpits that should have shed the water overboard.

Fittings Above the Waterline
It is also interesting to note that the finger was pointed at fittings above the waterline in 9% of the sinking claims. (Question: How can a fitting that is above the waterline sink a boat? Answer: Fittings that are above the waterline aren't always above the waterline.) More on this later.

Poor Docking Arrangements
Boats that sank after getting caught under a dock or banging against a piling accounted for 8% of the claims. This number did not include boats that sank during hurricanes, or the number would have been much higher.

Other
The only claim that didn't fit in any of the first four categories involved a boat that sank after lightning blew a transducer out.


Prevention: Protecting Your Boat

Visiting Your Boat: The First Line of Defense Against a Dockside Sinking

If you need a reason to visit your boat more often, consider that the cost of repairing a boat that has been underwater, even briefly, is usually about 40% of its value. Besides having to pay the deductible, the skipper typically loses the use of the boat for several weeks while it is being repaired.

At least twice a season, inspect any fittings above or below the waterline that could be letting water into the boat. All too often, skippers rely on bilge pumps to bail them out when they can’t visit their boats. The pump fails and the boat sinks. If you can’t visit your boat regularly, consider using a buddy system with other boat owners to watch each other’s boats. Another alternative is to ask your marina manager to keep an eye on the boat. Many marinas offer routine inspections, but usually at an extra cost.

Click on the links below for more information about prevention:

Outdrive Boots:
Eleven of the 100 boats sank because the rubber boots had deteriorated. According to experts, outdrive boots should be examined two or three times a year. Rubber that looks dried out and cracked (cracks are most likely to appear in the creases) needs replacing. If possible, store the outdrive down, which eliminates most creases and prolongs the life of the rubber. Finally, for whatever reason, muskrats and other water-swimming vermin like to chew on outdrive boots. "RO-PEL" a malodorous commercial product, is an effective deterrent (One source: United Spray Systems: 800-950-4883.)

Mufflers:
Backfiring can blow a hole in a plastic muffler. Corrosion can eat a hole in an metal muffler. One boat sank when a plug that was on the bottom of the muffler was removed and not replaced. Both the muffler and the exhaust hose should be inspected carefully.

Anti-Siphon Loops and Check Valves:
When a hose exits the hull above the waterline from a point somewhere below the waterline-at a bilge pump or head (toilet)-an anti-siphon loop or check valve should be used to prevent water from flowing back into the boat. With a loop, the peak must be well above the waterline. Many boats have sunk when they were lowered an inch or two and water poured through an anti-siphon loop that was at or near the waterline.

Cabin, Deck, and Scuppers:
Even aboard boats with cabins and self-draining cockpits, it isn't unusual to have a leak or two at hatches, ports, chain plates, etc. Caulking these leaks keeps water out of the bilge and also may prevent costly structural repairs later. Open boats and boats with especially low freeboard should be hauled for the winter in colder climates, as they are prone to being shoved underwater by snow and ice.

Five sinkings were caused by clogged scuppers. When scuppers are clogged with leaves or debris, water backs up and has a tendency to find a way into the bilge. Two other sinkings occurred because scuppers were cracked or broken scuppers and water leaked into the bilge.

Seacocks and Gate Valves:
According to voluntary industry standards, seacocks or gate valves, which can be closed in an emergency or when the skipper is away from the boat for extended periods, must be used at all thru-hulls below the heeled waterline. The valves and fittings must be made of bronze or Marelon®, which are not likely to break when struck accidentally with a foot or anchor. (RC Marine's Marelon® seacocks are the only plastic seacocks that meet the requirements of Underwriters Laboratories.)

Seacocks are widely regarded as being more reliable than gate valves. In an emergency, a quick glance at a seacock will tell you whether it is open or closed. With a gate valve, you can't tell. Gate vales also have a reputation for failing internally because the different metals-steel inside, bronze outside-aren't compatible. One of the sinking claims occurred when a bronze gate valve disintegrated and broke in two. A similar problem occurred with a bronze seacock. In both cases, the failure of the bronze would seem to indicate stray current. Look for a pinkish color on the bronze, which indicates corrosion.

Other thru-hulls that need inspecting periodically are transducers and raw-water intake strainers. Ice can bend a strainer that isn't winterized properly. You should either drain the bowl or fill it with antifreeze. Even if the seacock has been closed for the winter, water can enter the boat when the seacock is opened in the spring.

Removable transducers and impellers must be locked in place securely or they can work loose and sink the boat. One boat sank because the dummy plug for the speedo had not been locked down properly. Two boats sank because the fittings for the air conditioner intakes broke. Finally, one boat sank because a stern drive bolt through the transom was "weeping" a small quantity of water. The boat's owner apparently hadn't been aboard for many weeks or even months.

Three of the boats sank when hoses slipped off the seacocks' nipples. Hoses connected to the fittings must be double-clamped with stainless steel clamps. Rusted clamps should be replaced.

Another boat sank because a hose split. Hoses at thru-hulls should be the reinforced type, which is usually a heavy black hose. Lighter, unreinforced PVC hoses can (and do) rupture and crack. Check the entire length of the hose, as excessive heat from the engine or chemicals (bilge cleaners, battery acid, etc.) can cause isolated failures. A boat in Michigan failed because it was rubbing against the rough edge of a hole cut through the fiberglass. Replace any hoses that are suspect.

Thru-Hulls Above the Waterline:
As a general rule, a boat whose gunwale is close to the water (low freeboard) has a greater chance of sinking accidentally. A ski boat, for example, is more likely to be overcome by rainwater, a slow leak, or a following sea than a cruiser whose impressive hull towers far above the water.

But a boat is often much "closer" to the water than its freeboard would indicate. A cracked thru-hull at the boot stripe or a cutout at the transom for an outboard motor well that isn't protected by a splash guard means that, as a practical matter, the boat has to sink only an inch or two before it floods and heads to the bottom.

In one claim, holes at the boot stripe were meant to drain the deck of an open boat. Rain lowered the waterline and water began lapping onto the deck, which proved to be less than watertight. In another claim, snow shoved the drain of a 30' sailboat underwater. The plastic fitting had deteriorated in the sunlight and cracked, so that water was free to enter the boat.

Inspect fittings and hoses above the waterline with the same critical eye that you used on fittings down in the bilge. Double-clamp the thru-hulls and consider adding an anti-siphon loop or check valve to any that are within 8" to 12" of the waterline.

Dockside Freshwater System:
Four of the 100 boats sank because of problems in the boats' dockside freshwater systems. In one case, water entered through a broken fitting in the boat's hot water heater. Another sank after a hose burst (the freshwater system hadn't been properly winterized). The first line of defense against this sort of sinking is to turn off the water at the dock whenever you'll be away from the boat for more than a few hours. (There are also devices available at hardware stores that can be preset to shut off the water supply automatically.) Hoses and clamps throughout the system should be inspected periodically. While you're checking, make sure there's a pressure-reducer valve and only reinforced hose (look for the criss-cross pattern if the hose is made of clear PVC) is used, which accommodates the greatly increased pressure of a city water system.

Keep the Boat Away from the Dock:
Ten boats sank because they either got caught under the dock or banged against the dock. Bow, stern, and spring lines should be arranged to keep the boat in the center of its slip. Fenders and fenderboards can be used to cushion minor bumps but they will not overcome a poor docking arrangement. Double up on lines and use chafe guards if the boat is in an exposed location.

Why Boats Sink Underway

In addition to studying why boats sink at the dock, BoatUS Marine Insurance examined 50 claim files for boats that sank underway, ranging from a tiny personal watercraft to a 54’ ocean going sailboat. None of the 50 sinking claims involved fatalities, although that is always a possibility when a boat sinks with passengers aboard. One thing that became clear after reading the various claims: wearing life jackets or at least keeping them handy, should be a priority on any boat.

Any boat has the potential to sink underway for the same reasons that it could sink at the dock--a hose slips off, a packing gland leaks, etc. While Thirty four percent of the boats in the study sank because of leaks at thru-hulls, outdrive boots, or the raw water cooling system, all of which are routinely implicated when boats sink at the dock. There are many other reasons that boats sink underway, however, which have nothing to do with loose hose clamps or broken fittings. Boats underway can strike floating debris or stray onto a rocky shoal (“Navigation error”). There were claims for careless skippers who forgot to install drain plugs. Six percent of the boats sank after coming down hard off of waves and splitting open.

Once a boat starts to sink, it will gain momentum as it settles into the water. If a boat has a two-inch hole that is a foot below the waterline, for example, over 78 gallons of water will pour into the boat per minute. When the same hole is three feet below the surface, the flow of water increases to 136 gallons per minute. Keep in mind also, that other thru-hulls that had been above the waterline will be underwater. If any of these fittings are cracked or missing, the flow of water into the boat will accelerate further.

Why Boats Sink On Open Water
Click on a link below for more information:
Reason Percentage
30%
18%
12%
12%
10%
6%
4%
4%
4%

Taking Water Over the Gunwales:
The single most critical reason boats are flooded on open water has to do with transom height. Thirteen of 15 boats in the sample group that were swamped were outboard powered, with engine cut-outs that were often only inches above the waves. (Of the two remaining boats, one was an inboard with very low freeboard that took a wave over the bow and the other was a sailboat that was knocked down and sank when water entered a unsecured cockpit hatch.)

Motor wells are supposed to be the second line of defense when a wave comes over an outboard's transom but, in some cases, the well is too low, too shallow, and/ or not sealed adequately to the cockpit. Scuppers in the motor well and cockpit may also be slow to drain, especially if they re clogged. And whenever water lingers in the well or cockpit, the chances of another wave coming aboard increases. So too is the risk of being swamped.

Aside from transom height, the other contributing factor when a boat is swamped is typically weight distribution-- too many people at the stern together with scuba tanks, large coolers, bait wells, etc. that reduces buoyancy aft. In most cases, the boats were stopped or idling. The one exception was a boat that broached while entering a breaking inlet.

It should be noted that boats under 20' are required to have level flotation, so many of the boats in the study remained awash, although several were rolled over by the waves or by passengers rushing to one side of the boat.

Prevention: Especially on outboards with low cut outs, be conscious of weight distribution. Avoid storing scuba tanks, heavy coolers, etc. near the transom At slow speeds, keep the boat moving toward the waves. Don't anchor from the stern.

Most scuppers are slow to drain anyway, but when they're plugged up with leaves and other boat-gunk the water can linger in cockpits and motor wells a dangerously long time. Use a dockside hose with a power spray nozzle to flush out debris.

Leaks at Thru-hulls:
Any hole below the waterline has the potential to quickly sink a boat. In the BoatUS study, leaking thru-hulls included stuffing boxes, a bait well discharge, washdown fitting (softened by spilled fuel), knotmeter plug, bow thruster hose, broken scupper, and a failed head discharge. Unlike boats at a dock, which tend to sink as a result of relatively slow leaks at thru-hull fittings, these failures tended to be sudden, with many gallons of water pouring through the hole every minute. A 35' sailboat being en route from Maine to Maryland, for example, almost sank when a thru-hull knotmeter impeller, which hadn't been secured properly, popped out. The boat had motored many miles before the delivery skipper noticed that the boat seemed sluggish and finally checked below. He managed to save the boat, but water came within inches of sending it to the bottom.

Prevention: Any "opening" in the hull, whether it's protected by a seacock or stuffing box, needs to be inspected periodically. The same is true for openings that are slightly above the waterline. Seacocks and gate valves should be operable. Seacocks that are "frozen" open or shut should be taken apart and lubricated. Clamps (stainless steel) on hoses should be snug and free of rust. Two clamps are better than one. Hoses at thru-hulls should be the reinforced type, which is usually a heavy black hose. Lighter, unreinforced PVC hoses can (and do) rupture and crack. Check the entire length of the hose, as excessive heat from the engine or chemicals (bilge cleaners, battery acid, spilled fuel, etc.) can cause isolated failures. Replace hoses that are suspect--mushy, hard, and/or cracked. And, should all else fail, it's a good idea to tie a soft wood plug at every thru-hull.

Stuffing boxes, where the engine shaft exits the hull, are supposed to leak slightly underway but not when the boat is at rest. Check not only for leaks, but to make sure the packing material is firmly in place. Once the packing material is gone, water can pour into the boat. While less prone to failure, the stuffing box for the rudder should also be checked periodically.

Leaks at Raw water Cooling System/Exhaust:
A 300-hp engine pumps approximately 30 gallons of water through the cooling system every minute. Depending on which fitting lets go, you could find yourself with the water pouring into the bilge at the same time the engine overheats, which means you're liable to be greeted by clouds of hot steam when you open the engine hatch.

Which fittings are most vulnerable? Any fitting that is loose or corroded can let go. In one case a cooling water pump hadn't been adequately tightened. On other boats, hoses slipped off, a raw water heat exchanger burst (end cap), and a plastic muffler split open when the engine backfired.

Prevention: All of the fittings in the cooling system should be inspected periodically for loose connections and brittle or split hoses. Typically, a break in the cooling system will cause the engine to overheat before much water has been pumped overboard. The hatch is opened, the problem is discovered, and the boat can usually be saved. The exception is a break in the exhaust or muffler. Backfiring can blow a hole in a plastic muffler, corrosion can eat a hole in a metal muffler, exhaust hoses can split and the engine will continue to pump water--a lot of water--aboard.

Drain Plug Missing:
It's difficult to understand how a missing drain plug could sink a boat. Wouldn't the skipper realize that the boat was filling up with water? Typically, the water is out of sight in the bilge until hundreds of gallons have come aboard. By then, the boat might be floating well below its lines. In some cases, the source of the leak wasn't discovered until the boat was raised.

Prevention: How can an absent minded skipper remember to install a drain plug? Try leaving a drain plug (you should have at least one spare) with the trailer's winch handle or with the ignition key --anywhere it is sure to be seen before launching the boat.

Navigation Error (Grounding):
Despite all of the channel markers and buoys, not to mention electronics available to navigators, boats still go aground, sometimes with catastrophic results. Boats sank after striking underwater jetties, rocks, and, in one claim a soft bottom. The latter was a boat in California that lost its prop when it struck the bottom near a channel entrance. The boat drifted into the surf before its skipper could set an anchor.

Prevention: Charts, lorans, GPS, compasses, and even radar a are all available to help the navigator. Use them. If you're not sure where you are, slow down and keep an eye on the depth sounder.

Boat Construction (Hull Split Open):
Three of the boats--six percent-- sank because their hulls split open. In each claim, the cause of the failure was a lightly built hull slamming into waves or, in one case, another boat's wake.

Prevention: How can you tell if your boat was intended by the builder to withstand offshore conditions? A boat's weight, relative to other boats the same size and type, can give you a clue. So too can the quality of its hardware and finishing work. The best source for finding a boat's reputation, however, is other boat owners, marine surveyors, repairers, and the BoatUS Consumer Protection Bureau. The BoatUS web site (boatus.com) includes a "Boater to Boater Directory" that allows you to ask over 1,100 boat owners about specific make and model boats.

Don't depend solely on a manufacturers' promotional literature. A surveyor called Seaworthy recently to talk about a lawsuit that involved a boat whose hull split open after flying off of a large wave. The boat's manufacture claimed that the boat had been abused. The owner countered with a photo of the same model boat flying off a wave at high speed. The photo was being used in the manufacturer's promotional literature.

Leaks at Thru-hulls:
Any hole below the waterline has the potential to quickly sink a boat. In the BoatUS study, leaking thru-hulls included stuffing boxes, a bait well discharge, washdown fitting (softened by spilled fuel), knotmeter plug, bow thruster hose, broken scupper, and a failed head discharge. Unlike boats at a dock, which tend to sink as a result of relatively slow leaks at thru-hull fittings, these failures tended to be sudden, with many gallons of water pouring through the hole every minute. A 35' sailboat being en route from Maine to Maryland, for example, almost sank when a thru-hull knotmeter impeller, which hadn't been secured properly, popped out. The boat had motored many miles before the delivery skipper noticed that the boat seemed sluggish and finally checked below. He managed to save the boat, but water came within inches of sending it to the bottom.

Prevention: Any "opening" in the hull, whether it's protected by a seacock or stuffing box, needs to be inspected periodically. The same is true for openings that are slightly above the waterline. Seacocks and gate valves should be operable. Seacocks that are "frozen" open or shut should be taken apart and lubricated. Clamps (stainless steel) on hoses should be snug and free of rust. Two clamps are better than one. Hoses at thru-hulls should be the reinforced type, which is usually a heavy black hose. Lighter, unreinforced PVC hoses can (and do) rupture and crack. Check the entire length of the hose, as excessive heat from the engine or chemicals (bilge cleaners, battery acid, spilled fuel, etc.) can cause isolated failures. Replace hoses that are suspect--mushy, hard, and/or cracked. And, should all else fail, it's a good idea to tie a soft wood plug at every thru-hull.

Stuffing boxes, where the engine shaft exits the hull, are supposed to leak slightly underway but not when the boat is at rest. Check not only for leaks, but to make sure the packing material is firmly in place. Once the packing material is gone, water can pour into the boat. While less prone to failure, the stuffing box for the rudder should also be checked periodically.

Struck Floating Debris:
One boat, a 36' sailboat, sank as a result of striking a submerged object--it was never identified-- off the coast of Florida. Another boat struck a large log in an inlet on the Potomac River. Submerged or partially submerged boards, logs, etc., are typically swept into rivers and bays after large rain storms and have been responsible for damaging and even sinking many boats.

Prevention: Slow down whenever you see floating debris. For every log visible on top of the water, there is likely to be two that are bobbing just below the surface. If you do strike something, indicated by an ominous klunk somewhere on the hull, open the engine hatch immediately and make sure the boat isn't taking on water.

Other:
One boat was abandoned in a storm at sea and is presumed to have sunk. Another sank as the result of a faulty repair.

A Few Important Words About Pumps and Bilge Alarms

Two BoatUS Members, Cliff and Sandy Steele, tell a harrowing story about a log that almost sank their boat just before nightfall. After hearing a loud “thump,” Cliff checked the bilge and continued on. Sometime later the boat seemed to be losing power and felt sluggish, so he opened the hatch and discovered water was almost over his engine. Although Cliff had checked the bilge earlier, the damaged hull didn’t fail until it had pounded over some waves. Thanks to some nearby boaters who responded to Cliff’s “Mayday” by putting extra pumps aboard, the boat was saved.

The sooner a skipper discovers a leak down below, the more likely he or she will find and correct the problem before it’s too late. High capacity bilge pumps and even extra pumps can help in an emergency. So too can using the engine’s raw water intake hose (close the seacock first) for extra pumping capacity in an emergency.

While more and better pumps may be able to keep up with the flow of water, it is critical that you discover the leak quickly, before the electrical system, the engine, and the leak itself are underwater. A bilge alarm is a simple device that warns you when water begins rising in the bilge. This early warning gives you more time find the leak, get passengers into life vests, deploy extra pumps, and put out a distress call, Bilge alarms are available from most marine chandleries, including BoatUS.